Typically, a so-called HST (Hydro Static Transmission) is mounted in a work vehicle such as a wheel loader. An HST type of work vehicle drives a hydraulic pump using an engine and drives a movement hydraulic motor using hydraulic fluid which is discharged from the hydraulic pump. Due to this, the work vehicle moves. In such an HST type of work vehicle, it is possible to control vehicle speed and traction force by controlling the rotation speed of the engine, the displacement of the hydraulic pump, the displacement of the movement hydraulic motor, and the like (refer to Japan Patent Laid-open Patent Publication JP-A-2008-275012).
In the work vehicle, it is possible for an operator to select execution of traction control. The traction control is control where maximum traction force is reduced by the displacement of the movement hydraulic motor being limited to an upper limit displacement which is smaller than the maximum displacement. FIG. 10 shows an example of motor displacement and drive circuit pressure characteristics which regulate the drive circuit pressure and the displacement of the movement hydraulic motor. For example, when the upper limit displacement of the movement hydraulic motor is set as Ma, the maximum traction force is reduced more than a state where the traction control is not performed. In addition, when the upper limit displacement is set as Mb, the maximum traction force is further reduced.
On the other hand, the displacement of the hydraulic pump is controlled in the work vehicle based on pump displacement and drive circuit pressure characteristics as shown in FIG. 11. L11 to L16 in the diagram are lines which indicate the pump displacement and drive circuit pressure characteristics which change according to the engine rotation speed. The drive circuit pressure is fluid pressure of the hydraulic fluid which is sent from the hydraulic pump to the movement hydraulic motor. The pump displacement and drive circuit pressure characteristics change from L11 to L16 as the engine rotation speed increases. In addition, as shown by L12, the pump displacement is reduced as the drive circuit pressure increases and the pump displacement increases when the drive circuit pressure is reduced. A cut off valve is provided in the work vehicle so as to protect the movement hydraulic circuit. The cut off valve reduces the drive circuit pressure so that a predetermined cut off pressure value is not exceeded. As a result, as shown in L14 in FIG. 11, the cut of valve is activated in a case where the pump displacement is a predetermined pump displacement value Qx or less, the drive circuit pressure is reduced and becomes constant at a cut off pressure value Px.